1. Field of the Invention
The present invention relates to an electro-optic light shutter which is made of transparent PLZT ceramics, namely, (Pb, La) (Zr, Ti)O.sub.3 ceramics.
2. Description of the Prior Art
In general, a PLZT ceramic has the following advantages:
(i) it is transparent; PA1 (ii) its optical characteristic can be controlled electrically; and PA1 (iii) its electro-optic constant is large and an element can be formed with ease and at low manufacturing cost. PA1 a ceramic body having electro-optic effect; PA1 means to apply A.C. voltage to said ceramic body to cause electro-optic effect in said ceramic body; PA1 means to apply linear polarized light to said ceramic body; and PA1 means to detect light passed through said ceramic body, wherein said ceramic body is formed of PLZT ceramics having memory effect at room temperature and losing said memory effect and exhibiting quadratic electro-optic effect at an elevated operating temperature caused by application of A.C. voltage to said ceramic body. PA1 a ceramic body having electro-optic effect; PA1 means to apply A.C. voltage to said ceramic body to cause electro-optic effect in said ceramic body; PA1 means to apply linear polarized light to said ceramic body; PA1 means to detect light passed through said ceramic body; and PA1 means to hold said ceramic body at a predetermined elevated operating temperature, wherein said ceramic body is formed of PLZT ceramics having memory effect at room temperature and losing said memory effect and exhibiting quadratic electro-optic effect at said predetermined elevated operating temperature caused by application of A.C. voltage to said ceramic body and said means to hold said ceramic body at a predetermined elevated operating temperature.
FIG. 1 is the diagram showing a principle of construction of a PLZT electro-optic light shutter apparatus. In FIG. 1, reference numerals 1 and 2 respectively designate linear polarizers between which an electro-optic light shutter 3 formed of PLZT ceramics is disposed. The electro-optic light shutter 3 comprises a pair of electrodes 4 and 5 disposed on PLZT ceramics 6. The linear polarizers 1 and 2 are disposed in such a manner that the electric field vector directions of the linearly polarized light selected thereby may be perpendicular to each other. The PLZT ceramics of the shutter are disposed in such a manner that the electric field direction, and, the optic axis directions are at 45.degree. to each other. With such arrangement, when a light from the light source S of which the electric field vectors are oriented in every direction is passed through the polarizer 1, the light becomes linearly polarized light. The power of the linearly polarized light passed through the polarizer 1 is reduced to 1/2 that of the incident light according to theory. As the linearly polarized light passed through the polarizer 1 advances through the PLZT plate 6 of the shutter 3, the electric field vector of the polarized light is rotated depending on the applied voltage to the PLZT plate 6 and the plate thickness thereof. If the condition that the electric field vector is rotated by 90.degree. is set, the linearly polarized light emitted from the PLZT plate 6 is passed through the polarizer 2. When no voltage is applied to the PLZT plate 6, the light is in the crossed nicols condition so that the light can not pass through the polarizer 2, namely, the PLZT shutter 3 is in the off state, while when a predetermined voltage is applied to the PLZT plate 6, the light can pass through the PLZT shutter 3, namely, the PLZT shutter 3 is in the on-state. As described above, depending on the voltage applied to the PLZT shutter 3, the on and off condition of the light introduced thereto can be carried out whereby to perform the electro-optic shutter function.
Such PLZT electro-optic shutter has such advantages that it is high speed with response times of the order of micro seconds, made as a full solid state structure, long in life, high in reliability and so on. Accordingly, such a PLZT electro-optic shutter is now applied to, for example, flash brightness prevention shutter, a viewer in stereoscopic television system or a color television receiver employing a black and white television picture tube which was disclosed in Japanese patent application unexamined publication number 55685/1982 (corresponds to U.S. Pat. No. 4,490,739 or a high brightness projector and so on. The color television receiver disclosed in the above Japanese patent application unexamined publication number 55685/1982 will briefly be described hereinafter. At the external side of a face plate of a black and white (monochromatic) television picture tube is disposed a parallel stripe filter formed of red, green and blue color stripes, each being extended in, for example, the vertical direction. Electro-optic shutter elements are disposed at each stripe in opposing relation to the filter. In response to the scanning position of the electron beam at each horizontal scanning line of the picture tube, the electron beams are modulated by the color signals corresponding to red, green and blue colors and the electro-optic shutter element of each stripe is alternately turned on and off in synchronism therewith. Thus, each optical image of each of red, green and blue colors is produced through each stripe filter and hence a color reproduced image can be obtained as a whole.
FIG. 2 shows an example of a construction of a high brightness projector to which the above PLZT electro-optic shutter array is applied. Also in this case, when each element of the electro-optic light shutter array is turned on and off, a picture of one horizontal scanning line is formed sequentially. Namely, in this case, the light emitted from a light source 11 is projected through a collimator lens 12, an iris 13, a linear polarizer 14, a lens system 15 and a mirror 16 to an electro-optic light shutter 17. The light projected on the shutter 17 is made by the iris 13 and the lens system 15 into a light flux of a slit-shape, which slit-shaped flux extends perpendicular to the sheet of drawing of FIG. 2. The shutter 17 comprises PLZT elements which are disposed in stripe shape on a plane at the position irradiated by the above light flux of the slit-shape. Then, in response to the video signal of one horizontal scanning line amount, each element is sequentially turned on and off and the light passed therethrough is projected through an analyzer 18, a condenser lens 19, a movable mirror 20 and a projection lens 21 onto a screen (not shown). In this case, the polarizer 14 and the analyzer 18 respectively correspond to the polarizers 1 and 2 in FIG. 1. The movable mirror 20 is driven or vibrated along the surface of the sheet in FIG. 2, namely, in the horizontal direction shown by arrows, at, for example, one field period. On the other hand, when a video signal of one horizontal scanning line amount is sequentially supplied to each element of the shutter 17 and the movable mirror 20 is driven at each field period in synchronism therewith, a picture of one field amount is sequentially projected onto the screen.
In the color television receiver formed by the combination of the monochromatic cathode ray tube and the electro-optic light shutter or the high brightness projector employing the electro-optic shutter and so on, the driving of the shutter is carried out at a high driving frequency higher than 5 kHz.
PLZT is formed of a mixed system of PbZrO.sub.3, PbTiO.sub.3 and La.sub.2 O.sub.3 and the composition formula thereof is expressed by ##EQU1## Since its composition is determined by the amounts of La, Zr and Ti, its composition can be given by the expression of x/y/z. In accordance with the composition ratio of the x/y/z, the PLZT is roughly classified into a linear electro-optic effect material which shows a Pockels effect as a mode of birefringence caused by electro-optic effect, quadratic electro-optic effect material which shows a Kerr effect and a memory material which shows a memory effect. The quadratic electro-optic effect material, when applied with the electric field, causes anisotropy in permittivity and birefringence .DELTA..sub.n. However, if the electric field is removed therefrom, anisotropy in refractive index disappears and becomes isotropic. On the contrary, in the linear electro-optic effect material and the memory material, even if the electric field is removed therefrom, anisotropy remains therein. The relation between the birefringence .DELTA..sub.n and the electric field E of each material becomes as shown in FIGS. 3 to 5. These characteristics are shown at room temperature. FIG. 3 shows a case of the linear electro-optic effect having the composition of x/y/z=12/40/60, FIG. 4 shows a case of the quadratic electro-optic effect material having the composition of x/y/z=9/65/35 and FIG. 5 is a case of the memory material which has the memory effect having the composition of x/y/z=8/65/35. FIG. 6 shows the ranges of the compositions bounded by the solid lines which form the memory effect material, the linear electro-optic effect material and the quadratic electro-optic effect material having each composition of x/y/z. In FIG. 6, the value of y increases from right to left and the value z increases from left to right.
In the prior art, PLZT ceramics suitable for the electro-optic light shutter were selected to have the composition of 9.5 to 9.0/65/35 as the quadratic electro-optic effect material. The reason for this is that the PLZT having the above composition can present the quadratic effect at environmental temperatures ranging from -20.degree. C. to 80.degree. C. and can be used as the electro-optic light shutter. However, the temperature characteristic of the quadratic electro-optic coefficient R of the PLZT has a large negative value so that in the use at a temperature higher than room temperature the operation voltage is raised and hence the operational electric power is increased, a discharge between the electrodes occurs and so on, resulting in problems in the use.
On the other hand, when this PLZT is driven by A.C. current, the operational voltage is increased as its frequency is increased. In the graph of FIG. 7, curves 22, 23 and 24 indicate relations of operation voltage vs. output I/I.sub.0 (where I.sub.0 represents the incident light intensity and I represents the detected light intensity) in the case of D.C. operation, 500 Hz and 5 kHz.